Here are a few ideas that worked for me...
Playdoh. It is amazing how much fun a little tub can be with a plastic knife and a little tiny cookie cutter. Just open a napkin or two and lay it on the floor to catch any that might fall. (My kids were playing with playdoh at this age, but that homemade stuff you can eat will work too!)
Another thing I did was get a bunch of little gifts like from the dollar store and wrap them up in wrapping paper. eg...matchbox car, playdoh, little book, travel magna doodle, box of crayons, gummy snacks etc. It takes them 5 minutes to open the package and I found that you got about 20 minutes out of each gift depending...that adds up! Depending on your flight, wrap about 3 gifts per hour. It is so much fun to open a new present every 20 minutes!
I agree that a portable DVD player is a WISE investment! If you can bum a Game Boy advance or a PSP player off someone, you can get movies for them too and they are much smaller!
CU SOON!